Check whether your phone supports the older WhatsApp version you want
Back up your chats before changing the app
Uninstall the current WhatsApp version
Download the older WhatsApp APK from a trusted source if you use Android
Enable installation from unknown sources in your phone settings if required
Install the APK file
Open WhatsApp and verify your phone number
Restore your chat backup if prompted
Turn off automatic app updates in the Play Store if you want to keep the old version
Use an iPhone app backup or restore method if you need an older iOS version, since direct downgrades are usually not available
Make sure the version you install is compatible with your device and operating system